As light-emitting diodes are becoming cheaper and more attractive for use in retrofit lighting applications, low-cost driver solutions are required. Various driver topologies are known from the prior art, for example as described in U.S. Pat. No. 8,698,407 B1, US 2014/0049730 A1, US 2007/0097043 A1, US 2012/0181940 A1 and US 2010/0156324 A1.
In a retrofit LED lighting product, one or more power LEDs are incorporated together with driver circuitry in a standard light-bulb fitting, for example a GU10 fitting. There are a number of ways of powering a retrofit LED lamp from a mains power supply or power converter. However, since some degree of flexibility is desirable to manage different forward voltages of different LEDs, the driver circuitry is usually based on a switched-mode power supply (SMPS). For example, the driver can comprise a single-stage self-oscillating SMPS.
Preferably, a retrofit LED lighting application should be able to be used in conjunction with an already existing dimmer. A widely used type of legacy leading-edge dimmer is usually arranged to follow a rectifier and operates by performing phase cut on the leading portion of the rectified mains voltage. The “phase cut angle” refers to the angle (between 0° and 180°) up to which the dimmer suppresses or cuts a half-wave of the rectified mains voltage signal. An LED lamp driver, which must be arranged between such a legacy dimmer and the LEDs, receives this phase-cut input voltage and must be able to fulfill certain requirements such as providing a minimum holding current. This can be achieved for example by a power converter with a buffer capacitor connected across the output of a switched-mode power supply. The buffer capacitor smoothens the phase-cut voltage at the output of the driver, but voltage ripple on the buffer capacitor will result in some level of ripple on the LED current. During an undimmed or only slightly dimmed mode of operation of the LEDs (small phase cut angle), the LED current is relatively high, so that the current ripple effect is less compared to dimmed mode, for example at a 90° conduction angle, and does not adversely influence the light output. However, with increasing LED efficiency, the equivalent series resistance (ESR) of power LEDs is decreasing, and the ESR of an LED is roughly inversely proportional to its current rating. For an LED with a low ESR, therefore, the voltage ripple on the buffer capacitor will have a significant impact on the LED current ripple, which reaches a maximum at phase cut angles of about 90° and which can cause noticeable flicker on the light output of the LEDs.
One way of reducing the LED current ripple with the aim of suppressing flicker might be to use a larger buffer capacitor. However, since the driver and LEDs are usually implemented in a retrofit light-bulb product, the physical bulb dimensions present design constraints that may rule out the use of a larger capacitor, since physical size generally increases with capacitance. Another way of dealing with the undesirable ripple might be to use an additional power-dissipating resistor in series with the LEDs. However, such a power-dissipating resistor lowers the efficiency of the lighting circuit due to power losses when the LEDs are driven in an undimmed mode of operation. A power-dissipating resistor is therefore unattractive from an environmental point of view, since the trend is towards more energy-efficient lighting solutions.
Therefore, it is an object of the invention to provide an improved way of driving an LED arrangement from a dimmable power supply while avoiding the problems described above.